“Cyclometalated iridium complex” is a general term for organic iridium complexes, in which multidentate ligands are coordinated to the iridium atom to form a ring, and at least one iridium-carbon bond is present, for example, tris(2-phenylpyridine)iridium [Ir(ppy)3] (Chemical Formula 1). Among cyclometalated iridium complexes coordinated with, as a ligand, an aromatic heterocyclic bidentate ligand such as a 2-phenylpyridine derivative, a 2-phenylquinoline derivative, or a 1-phenylisoquinoline derivative as in Chemical Formula 1 are used as phosphorescent materials for organic electroluminescent (EL) devices, organic electrochemiluminescent (ECL) devices, and the like (Patent Document 1). Phosphorescent materials have light-emitting efficiency about 3 to 4 times higher than that of fluorescent materials conventionally used in the development of organic EL devices, etc., and thus are expected to be put into practical use to achieve higher efficiency/energy saving.

Examples of cyclometalated iridium complexes include a biscyclometalated iridium complex, in which two aromatic heterocyclic bidentate ligands are coordinated to the iridium atom, and a triscyclometalated iridium complex, in which three aromatic heterocyclic bidentate ligands are coordinated to the iridium atom. Among them, triscyclometalated iridium complexes have particularly high thermal stability and, when applied to organic EL devices, etc., are expected to increase the life.
Such a cyclometalated iridium complex can be synthesized in a single step, for example, by allowing iridium trichloride as a raw material to react with an aromatic heterocyclic bidentate ligand such as 2-phenylpyridine (ppy) (Chemical Formula 2, Non-Patent Document 1). Additionally, a cyclometalated iridium complex can be obtained in a single step by allowing, as a raw material, tris(2,4-pentanedionato)iridium(III) (hereinafter sometimes referred to as Ir(acac)3), in which three 2,4-pentanediones are coordinated to iridium, to react with an aromatic heterocyclic bidentate ligand such as 2-phenylpyridine (ppy) (Chemical Formula 3, Non-Patent Document 2). Further, Patent Document 2 discloses a method including allowing iridium trichloride as a raw material to react with an aromatic heterocyclic bidentate ligand such as 2-phenylpyridine (ppy) to perform multi-step synthesis method via a chlorine-crosslinked dimer (Chemical Formula 4).

However, a cyclometalated iridium complex obtained by single-step synthesis using iridium trichloride as a raw material as in Non-Patent Document 1 is problematic in that a chlorine component derived from iridium trichloride remains in the cyclometalated iridium complex. It has been pointed out that such a chlorine component adversely affects the light-emitting properties when applied to an organic EL device (Patent Document 3).
Meanwhile, in Non-Patent Document 2, non-chlorine tris(2,4-pentanedionato)iridium(III) is used as a raw material, and thus a chlorine component derived from the iridium raw material does not remain at all. However, tris(2,4-pentanedionato)iridium(III) is thermally stable and has low reactivity, and thus has been problematic in that the synthesis yield of the cyclometalated iridium complex is low.
Specifically, because tris(2,4-pentanedionato)iridium(III) is thermally stable, in order to obtain a cyclometalated iridium complex in good yield, generally, the synthesis is performed under high-temperature conditions at 200° C. or more. Therefore, sometimes, an unexpected decomposition reaction proceeds, and the yield or purity was reduced. Additionally, it has been pointed out that due to 2,4-pentanedione produced with the progress of the reaction, the temperature in the reaction solution becomes less likely to rise sufficiently, causing a decrease in the yield of the cyclometalated iridium complex (Patent Document 4).
Additionally, tris(2,4-pentanedionato)iridium(III) has symmetric R-diketone ligands, and thus has excellent crystallinity and is in a solid state at room temperature. Such tris(2,4-pentanedionato)iridium(III) in a solid state has sublimability. According to the findings of the present inventors, in the course of producing a cyclometalated iridium complex, tris(2,4-pentanedionato)iridium(III) may deposit on the top of the reaction vessel and come out of the reaction system. This is considered to be another cause of a decrease in the yield of the cyclometalated iridium complex.
Therefore, when a cyclometalated iridium complex is obtained using tris(2,4-pentanedionato)iridium(III) as a raw material, in order to improve the yield of the cyclometalated iridium complex, adding a reaction promoter to the reaction system is proposed. Patent Document 5 and Patent Document 3 describes the addition of a Lewis acid and a Bronsted acid, respectively, as a reaction promoter to the reaction system in obtaining a cyclometalated iridium complex.
However, the production methods described in Patent Document 3 and Patent Document 5 have fundamental problems in that they cannot be applied when aromatic heterocyclic bidentate ligands or the reaction product is unstable to acids. Therefore, with these production methods, the yield of the cyclometalated iridium complex cannot be necessarily improved sufficiently, and the development of a novel production method has been craved. Further, the production method disclosed in Patent Document 2 is a multi-step synthesis method, and thus takes time and effort and also requires the isolation/purification of the product in each step. Therefore, there are disadvantages in terms of production cost.